1. Field of the Invention
The present invention relates to phase-locked loop (PLL) circuits, and, more particularly, to controlling a profile of the PLL circuit's output frequency spectrum.
2. Description of the Related Art
In many electronics applications, digital circuits are employed that operate with one or more clock signals. Personal computers commonly employ a processor that may operate based on a clock having a frequency of 350 MHz or more. However, at such high frequencies, these digital circuits may radiate signals as electromagnetic energy, and these electromagnetic emissions may interfere with the operation of surrounding equipment. Since these emissions are based upon clock signals, high emitted energy “spikes” occur at these clock signal frequencies and their harmonics. Consequently, equipment is often shielded to prevent or minimize these emissions within certain frequency ranges, or operation is modified to spread the emitted energy over a wider frequency range, thereby decreasing the energy at any given frequency. One technique for modifying the operation of a digital circuit is to vary the clock frequency over a range of frequencies such that the average frequency is the desired clock frequency, but the emitted energy is now “spread” over the range of frequencies. Such variation of the clock is termed “spread spectrum” and reduces the interference from high energy spikes at the clock frequency.
A synthesizer generating one or more clock signals often employs a phase-locked loop (PLL). A PLL is a circuit that generates a periodic output signal that has a constant phase and frequency with respect to a periodic input signal. PLLs are widely used in many types of measurement, microprocessor, and communication applications. One type of phase-locked loop is the charge-pump PLL, which is described in Floyd M. Gardner, “Charge-Pump Phase-Lock Loops” IEEE Trans. Commun., vol. COM-28, pp. 1849-1858, November 1980, the teachings of which are incorporated herein by reference. In many applications, the frequency of the output signal is higher than the frequency of the input signal.
In a conventional charge-pump phase-locked loop, a phase detector (PD) compares the phase θIN of the input reference clock signal to the phase θOUT of a feedback signal derived from the PLL output. Based on the comparison, the PD generates an error signal: either an UP signal (when θIN leads θOUT) or a DOWN signal (when θOUT leads θIN), where the error signal indicates the difference between θIN and θOUT. A charge pump generates an amount of charge equivalent to the error signal from the PD, where the sign of that charge indicates the direction of UP or DOWN. Depending on whether the error signal was an UP signal or a DOWN signal, the charge is either added to or subtracted from the capacitance in a loop filter. As such, the loop filter operates as an integrator that accumulates the net charge from the charge pump. The resulting loop-filter voltage VLF is applied to a voltage-controlled oscillator (VCO). A voltage-controlled oscillator is a device that generates a periodic output signal, whose frequency is a function of the VCO input voltage. Input and feedback dividers may be placed in the input and feedback paths, respectively, if the frequency of the output signal is to be either a fraction or a multiple of the frequency of the input signal.
One method of spread spectrum to vary a clock frequency employs modification of the feedback divider used to control the output clock frequency of the PLL. The feedback divider typically divides the output signal of the VCO by a fixed number N to generate a signal close, in frequency, to the input reference clock signal. By varying the value of N, the divided output of the VCO applied to the phase detector also varies the output frequency of the VCO. Spread spectrum techniques of the prior art typically vary the frequency in discrete steps by reading successive values for N from a table stored in memory and supplying the successive values of N to the feedback divider.